1. Field of the Invention
Mining operations typically remove refuse, or gangue, along with the desired mine products, i.e., coal, ore, or other minerals. The device of this invention is used to separate such refuse from the desired products of the mine. Nine principles underlie this invention: First, the densities of desired mine products usually are different from the densities of the associated refuse components. Second, numerous pure heavy liquids exist which have densities intermediate between the densities of the mine products and of their associated refuse. Other densities can be achieved by mixing pure heavy liquids. Simply by density differences, sink and float fractions can be separated, thereby removing the mine products from the refuse. Third, in a modification of the second concept, where simple sink/float separations are ineffective the heavy liquids can be stirred to suspend one fraction in a liquid of slightly less density while a second, more dense fraction settles. Fourth, these heavy liquids are unable to wet into and fill the pores of solids if the liquids are at their boiling points and the solids are even hotter. Therefore, hot solids with their surfaces cooled to the heavy-liquid boiling point can be readily and essentially completely freed of clinging heavy liquids by drainage plus minor further heating to vaporize remaining liquids. Heating the liquids also assists the drainage because the viscosities and surface tensions of the liquids fall with temperature increases. Fifth, many suitable liquids do have boiling points which can be reached without damaging the products which are sought from mines. Sixth, the great density of the vapors of heavy liquids lets even hot vapors settle in a container of suitable design. Seventh, preheating the product/refuse mixture eliminates moisture which otherwise would (a) interfere with dispersion and separation of the product and refuse in the heavy liquid, (b) lead to metal corrosion, (c) require unnecessary product shipping costs, and (d) with coal, reduce the Btu output per pound of coal burned. Eighth, preheating the mine product, together with the thermal shock as the product enters the cooler bath of heavy liquid, separates product materials from refuse. Such refuse can be particularly undesirable. For example, the thermal shock breaks pyrite particles from coal, and sometimes it breaks the coal along pyrite streaks so that this previously trapped pyrite is exposed and can break off, thereby reducing sulfur in the coal. Ninth, elemental sulfur is dissolved away from coal by many of these heavy liquids.
2. Prior Art
A. "Application of Heavy-Liquid Processes to Mineral Benefication," by L. A. Roe and E. C. Tveter, Society of Mining Engineers Transactions, June, 1963, pp. 141-146. This article reviews the patent literature and processes prior to late 1962 and reports the failure of a coal plant in which 20,000 tons of coal were washed (separated) by heavy liquids, including trichloroethylene. From the standpoints of safety, environment, and economics, this approach to minerals beneficiation would still be unacceptable today. A second plant failed to recover carbon tetrachloride (CCl.sub.4) from purified clay. Preheating the clay, as is incorporated in the present invention, would have permitted CCl.sub.4 recovery and prevented the failure.
The patent summaries in the article show (a) evaporation recovery of volatile heavy liquids used for parting of mine product and reject fractions, (b) preconditioning of the mine product/refuse mixtures by coating with liquids or solids which are immiscible in the heavy liquids, and (c) the use of continuous conveyors (chains and spiral blades) for removal of the parted mine products and reject. However, these patents do not address the major advantages of predrying and preheating the mine product/refuse mixture to temperatures above the boiling point of the heavy parting liquids as shown in this invention. This preheating permits rapid completion of the heavy liquid recovery by drainage and vaporization; such preheating is essential if one is to operate an efficient, compact, and portable mineral washer (separator) which does not use process water.
B. "Demonstration Plant Test Results of the Otiska Process Heavy Liquid Benefication of Coal," by D. V. Keller, Jr., C. D. Smith, and E. F. Burch, presented to the Annual SME-AIME Conference, Atlanta, Ga, Mar. 7, 1977. This publication describes a large, stationary, pilot plant for separating coal from reject by sink/float separation in a heavy medium.
Because the Otiska people did not recognize the importance of preheating their coal/refuse mixture, they selected a process with major disadvantages. Their paper states that their choice of heavy liquid reflects "two key characteristics," namely that "(1) the heavy liquid does not react significantly with the coal product or reject material, and (2) the liquid permits complete dispersion of the coal product particles throughout the separation bath." To obtain these "key characteristics," they chose a heavy liquid which requires (a) system pressurization, a very difficult engineering problem, (b) refrigerated condensation, a large expense, and (c) coal pretreatment to allow coal particle dispersion, an unreliable process which must be tailored to each specific coal field. In addition, their system still requires heating the fractions, but this heating is done after the parting when the heating is more difficult and when it delays the processing of further material. Finally, the Otiska process is limited to only a very few of the many otherwise possible parting liquids. In the present invention a much simpler separation system is described which can be mounted on trailers and therefore is mobile.
3. Utility
The mineral separator of this invention provides a new method of purifying mine products, i.e., coal, ore, and minerals, from their associated refuse. Mine product/refuse mixtures are found in the new output of mines and in the reject piles from earlier mine and mine-mill operations. Separation and removal of such refuse from mining operations is necessary or advantageous both for transportation and for later processing and use of the mine products. Furthermore, many old reject piles present environmental hazards yet also contain materials which now would be valuable if they were separated from their associated refuse--there is incentive to reprocess such piles. This invention will provide an economically competitive separation of many such mine product/refuse mixtures. One particularly undesirable component of the refuse in coal mixtures is the sulfur, both elemental and as pyrites. The mineral separator of this invention offers significant removal of these forms of sulfur which are frequently present in coal. The mobility feature of this mineral separator is an especially valuable asset in dealing with old reject piles. Often the amount of valuable resource in such reject piles does not warrant construction of a permanent new mill, yet a mobile unit could move in, reprocess the reject piles, and move one to another site, thereby recovering mine products and reducing environmental problems. The waterless feature of this mineral separator is also important. Water rights which were available for mining in earlier days now in most cases been diverted to other uses. Also many potential mines have had to be abandoned because water was not available. Even where water is now available for mineral processing, water-based mineral processing creates pollution problems which are avoided by the mineral separator of this invention. The simplicity of this mineral separator is also economically important and eliminates many complex operations of some mine mills, e.g., froth flotation, magnetite recovery, filtration, cycloning, and waste settling. The separator of this invention avoids the pressurized operations and refrigeration of the separator discussed in Item B of Prior Art. As a consequence both of the new method of mine product recovery and of the corollary simplification of the equipment required, the equipment of this invention can be cheaper, fewer operators are required, and repairs are easier than for earlier separation equipment.